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19 Answers
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For attempting to "referee" an old-school or classic-style RPG game (based on or close to Basic D&D, Castles & Crusades, Swords & Wizardry, etc.), I'd definitely give a look at Matt Finch's Primer for Old School Gaming. It does a nice job of explaining some of the differences in philosophy between newer gaming and the classical assumptions some use for the game.

For D&D 3.x/Pathfinder, you can't go far wrong with the Pathfinder Gamemastery Guide. It's a number of essays, pieces of advice, and world/campaign-building tips that build on more than 10 years' experience with a basic system, and many more years of general gaming besides. This is one of the higher-rated books I've seen come out of Paizo, and that's saying something.

If you're more interested in "tools" and charts, and count those as advice, I would submit it would be well worth your time to find a copy of the Hackmaster 4th Edition GM's Guide. This book is chock full of charts, tables, and items for every single occasion. I use it with other games all the time, although it was written for Hackmaster. Again, this is less meta "advice" and more a toolkit for use in the game.

As an overall pick, as a generic product, I feel I would be remiss if I did not mention Robin's Laws of Good Gamemastering. Although not a system-specific product, it stands as an excellent resource of general GM advice, and is generally considered a classic treatment on the subject.

Over The Edge has one of the best description of how to run the game. It is an amazing game, full of story ideas (and not just for OTE), a gazillion NPCs, and locations. The "How to run a game" section is one of the best I have read. It is full of advise on how to make games more immersive, how to deal with players' unexpected antics, and how to run good multi layered games.

Spirit of the Century, a Fate-based game by Evil Hat, contains a lengthy and very useful section on how to run a game. It's broad enough that it applies to any genre and is helpful even if you're not running a Fate game. I re-read it from time to time to get inspired and to remember that more work doesn't mean better gamemastering.

Andy Kitkowski, creator of the Indie RPG Awards, said this about it:

Out of all the mainstream or indie books I’ve ever owned, no other
company or designer has ever, EVER put as much time, thought and
energy into making such a helpful section. The advice is universal
(save for the one section on specific pulp games), it is solid, it is
a MACHINE for producing GOOD GMs.

It's a hidden gem. I owned SotC for quite a while without having read it fully (since I wasn't planning on running it any time soon), and only discovered the section after reading about it online. It's pure gold.
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SevenSidedDie♦Mar 12 '13 at 19:55

Agreed. Spirit of the Century pretty much changed how I look at RPGs, both as a GM and as a player. And I don't even play it!
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Steve GMar 12 '13 at 20:05

I think that the best rule book for game master I have ever read is The Book of Mirrors from Mage: The Ascension.

It goes from advising about how to make a good story and what is the objective of doing so to how to game master people with serious issues.
Finally, this book it’s not about what you say during the game play, but how you focus so the game play is a good experience.

Mouse Guard is VERY well set up for the novice MG GM. It's highly structured, so it's easy to GM, and it has copious advice for the MG GM on how to build, use, and twist the structure.

Burning Empires also has excellent GM advice, and again, is a structured play style with a much more limited role than normal.

Both are written by Luke Crane. Burning Wheel, his magnum opus, is not as good on the accessibility score; BE and MG are written more carefully and with more advice, especially important because of their divergent play-styles.

A lot of great answers here, but I also enjoyed the gamemastering advice in John Wick's Houses of the Blooded. Wick advocates giving the players some game narration abilities, and his supporting ideas and examples in the Narrator chapter are excellent.

Hero Quest II (Robin Laws, 2009, Moon Design publications; I think you can get it as a PDF) has, unsurprisingly, a lot of GM advice, including a fair few ideas not in Robin's Laws of Good Gamemastering. I'm not a huge fan of HQ, the system, but the book has been well worth the price to me. HQ2 solves some issues from HQ1, btw, and has been made into a generic RPG with this book, together with a "Gaming in Glorantha" section that is, I think, mostly the work of Jeff Richard.

The new ideas here won't be a huge surprise to anyone who reads Robin's blog: the Playing Stories section, in particular, has a lot of the Beowulf saga that he had there. But the section is now a masterpiece, though.

I like Unknown Armies because it shows how to construct a relationship map and explains the value in doing so. I also found the information on the use of themes, plot development, hooks, motifs, descriptive language, and characterisation to be well-written, clear, substantive and practical. The GM material comes across as practical and usable, whereas I find many GM chapters in other games tend to be vacuous and too abstract. I would recommend it highly to a beginner, or someone without a background in some sort of literary study, and I would consider it very valuable to even an experienced DM as a refresher, reminder, and useful summation of certain points.

Not only is B1 In Search of the Unknown probably the best introductory module for Dungeons & Dragons ever published, it is one of the very few D&D modules that also has extensive notes for the fledgling DM. The sections "Notes for the Dungeon Master", "Preparation for the Use of the Module", "Time", "Computing Experience", and "How to be an Effective Dungeon Master" are one of the best five page tutorials for running ANY edition of the game. Some of the advice is really good for any type of RPG, not just D&D.

Some snippets:

"First, it is crucial to keep in mind that this is a game based on player interaction and player choice. The game generally follows the course of the player's actions—if not always their plans!"

"Although you may set up situations to challenge them, you must understand that you are not their adversary, nor are you necessarily out to "defeat" them"

"if your players abandon caution or make stupid mistakes, let
them pay the price—--but be fair."

The material in the D&D 4e Dungeon Master's Guide 2 is really good; it's full of concrete suggestions and examples. It's aimed at more experienced GMs who've run games before, rather than first-time GMs. Robin Laws wrote it, and he explains not just what you can do but why you might want to do it. This makes it a toolbox rather than a set of instructions. Most of it is good for any game. It does lean somewhat towards the encounter structure of 4e.

Over the Edge also has excellent advice. It's a more free-form game, to a degree that was really unusual at the time it was published, so the GM advice is great as an introduction to a more experimental style of play. For example, there's no skill or stat list. One of the examples is a character who abused that freedom to make an overpowered character, and how the GM can work with that impulse rather than quashing it. The advice about adlibbing adventures encapsulates a similar spirit and is equally useful.

Matthew Finch's Quick Primer for Old School Gaming is absolutely awesome for anyone who wants to understand or play in the old school style. It's not a game, but it's still very good. It'll help both players and GMs understand how to make decisions about PC actions when the rules don't cover a specific situation.

As someone noted above, anything by Vincent Baker has excellent GMing advice which, while certainly geared towards the game at hand, will always offer broader insight.

The sadly overlooked Prince Valiant by Greg Stafford is a treasure trove of clear, straightforward advice.

I would also include the supplement Sorcerer & Sword by Ron Edwards. While focused on the Sword & Sorcery genre in relation to his game Sorcerer, there's lots of great stuff in it about how to make play come alive for players.

Good call. Why not the other two Sorcerer supplements too? They're all three full of neat stuff.
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clweeksAug 23 '10 at 16:54

Indeed they are, though in my experience they're a bit further out to left field than some are comfortable with. Sorcerer & Sex, though, is possibly one of the most unique rpg texts I've ever seen...
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kesherAug 24 '10 at 13:54

I agree strongly with all your recommendations; they're all recommendations I would have made in my own answer. I'd add that the various incarnations of Pendragon (which Greg had a strong hand in) and HeroQuest (which Greg had a strong hand in) are also written with lots of good advice for everyone at the table on how to play the game (much of which can be extrapolated to games in general).
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Viktor HaagAug 31 '10 at 15:19

Ken Hite's Gurps Horror (and its earlier renditions) is some of the best genre-specific game master advice out there for running Horror.

It starts by laying out exactly what is and what isn't horror, going over various tropes and stylistic elements. It then gives solid advice for how to run these, both mechanically and stylistically. It then ends with some very nice campaign frameworks (and even gives a format for campaign frameworks that ahs served me well to this day).

The only other gamemaster advice that I've felt has come close are the two 4e Dungeon Master's guides.

Call me an old schooler but the 1st edition Dungeons Masters Guide is a treasure trove ideas and suggestions for game mastering. Granted it is not laid out in one convenient advice section, but spread thought the text. Even today after over thirty years of gaming I can pick up the 1st edition DMG page though it and almost always find a new nugget of wisdom or wisp inspiration that can help make my game a better one.

Apocalypse World. Hands down. Vincent has been refining the notion of teaching the players of his games how to actually play the game -- as opposed to just presenting a bunch of rules and then hoping the players get it. And thus far, Apocalypse World is his masterpiece. It is clear that it is teaching you one way to run games and then provides a ton of very specific things to do in order to run the game the way he wants you to.

Really, on reading this and thinking about it, it's kind of amazing that this is a new technology. But it is. And it's remarkable how different it is from most games in this regard.

Each one of Vincent's games gets better and better at precisely this. I found In a Wicked Age to be a wonderful, concise, game that showed you how to play and taught you how to play. I'm still reading through Apocalypse World, but it seems to be, as you say, an even better example in this mode.
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Viktor HaagAug 31 '10 at 15:21

I was very impressed with the GM's Survival Guide for "Legend of the Five Rings". It did a very good job of how to set tone, how to handle situations, etc. I would recommend it even to people who don't play Lo5R, simply because most of the advice is very system agnostic...it's just good GM ideas.